The present invention relates generally to metal and pseudometallic films suitable for implantation into mammalian subjects in need thereof. More particularly, the present invention pertains to prosthetic cardiac and venous valve implants, access ports and other implantable medical devices that employ moveable valve flaps. The implantable medical devices according to the present invention have improved valve flap members fabricated from metal and/or pseudometallic materials. It is desirable, although not essential to the present invention, that the prosthetic cardiac and venous valve implants be capable of being delivered using endovascular techniques and being implanted at an intracardiac or intravenous site without the need for anatomic valve removal. Embodiments of the prosthetic valves of the present invention are well-suited for cardiac delivery via a femoral or subclavian artery approach using a delivery catheter, and, depending upon the specific configuration selected, may be deployed within the heart to repair valve defects or disease or septal defects or disease. According to one embodiment of the invention, there is provided a chamber-to-vessel (CV) configuration that is particularly well suited as an aortic valve prosthesis to facilitate blood flow from the left ventricle to the aorta. In a second embodiment, there is provided a prosthetic valve in a chamber-to-chamber (CC) configuration that is particularly well-adapted for mitral valve replacement or repair of septal defects. Finally, a third embodiment is provided in a vessel-to-vessel (VV) configuration, which is well suited for venous valve exclusion and replacement.
Common to each of the CV, CC and VV embodiments of the present invention are a stent support member, a graft member which covers at least a portion of either or both the luminal and abluminal surfaces of the stent and valve flap members. Both the graft member and the valve flap members are preferably fabricated from metallic and/or pseudometallic materials, the valve flaps being coupled to the stent in a manner which biases the valve flaps so they close upon a zero pressure differential across the valve region.
More specifically, the valve flap members and the graft members of the present invention are fabricated entirely of self-supporting films made of biocompatible metals or biocompatible pseudometals. For purposes of this application, the term “pseudometal” or “pseudometallic” is intended to mean a biocompatible material which exhibits biological response and material characteristics substantially the same as biocompatible metals, such as for example composite materials.
As opposed to wrought materials that are made of a single metal or alloy, the inventive valve flap members and graft members are made of at least two layers formed upon one another into a self-supporting laminate structure. Laminate structures are generally known to increase the mechanical strength of sheet materials, such as wood or paper products. Laminates are used in the field of thin film fabrication also to increase the mechanical properties of the thin film, specifically hardness and toughness. Laminate metal foils have not been used or developed because the standard metal forming technologies, such as rolling and extrusion, for example, do not readily lend themselves to producing laminate structures. Vacuum deposition technologies can be developed to yield laminate metal structures with improved mechanical properties. In addition, laminate structures can be designed to provide special qualities by including layers that have special properties such as superelasticity, shape memory, radio-opacity, corrosion resistance etc.
It is important for the present invention to provide orientational definitions. For purposes of the present invention, references to positional aspects of the present invention will be defined relative to the directional flow vector of blood flow through the implantable device. Thus, the term “proximal” is intended to mean on the inflow or upstream flow side of the device, while “distal” is intended to mean on the outflow or downstream flow side of the device. With respect to the catheter delivery system described herein, the term “proximal” is intended to mean toward the operator end of the catheter, while the term “distal” is intended to mean toward the terminal end or device-carrying end of the catheter.
Conventional metal foils, wires and thin-walled seamless tubes are typically produced from ingots in a series of hot or cold forming steps that include some combination of rolling, pulling, extrusion and other similar processes. Each of these processing steps is accompanied by auxiliary steps that include cleaning the surfaces of the material of foreign material residues deposited on the material by the tooling and lubricants used in the metal forming processes. Additionally, chemical interaction with tooling and lubricant materials and ambient gases also introduces contaminants. Some residue will still usually remain on the surface of the formed material, and there is a high probability that these contaminating residues become incorporated during subsequent processing steps into the bulk of the wrought metal product. With decreasing material product size, the significance of such contaminating impurities increases. Specifically, a greater number of process steps, and, therefore, a greater probability for introducing contaminants, are required to produce smaller product sizes. Moreover, with decreasing product size, the relative size of non-metal or other foreign inclusions becomes larger. This effect is particularly important for material thicknesses that are comparable to the grain or inclusion size. For example, austenitic stainless steels have typical grain sizes on the order of magnitude of 10–100 micrometer. When a wire or foil with a thickness in this range is produced, there is significant probability that some grain boundaries or defects will extend across a large portion or even across the total thickness of the product. Such products will have locally diminished mechanical and corrosion resistance properties. While corrosion resistance is remedied by surface treatments such as electropolishing, the mechanical properties are more difficult to control.
The mechanical properties of metals depend significantly on their microstructure. The forming and shaping processes used to fabricate metal foils, wires and thin-walled seamless tubes involves heavy deformation of a bulk material, which results in a heavily strained and deformed grain structure. Even though annealing treatments may partially alleviate the grain deformation, it is typically impossible to revert to well-rounded grain structure and a large range of grain sizes is a common result. The end result of conventional forming and shaping processes, coupled with annealing, typically results in non-uniform grain structure and less favorable mechanical properties in smaller sized wrought metal products. It is possible, therefore, to produce high quality homogeneous materials for special purposes, such as micromechanical devices and medical devices, using vacuum deposition technologies.
In vacuum deposition technologies, materials are formed directly in the desired geometry, e.g., planar, tubular, etc. The common principle of the vacuum deposition processes is to take a material in a minimally processed form, such as pellets or thick foils (the source material) and atomize them. Atomization may be carried out using heat, as is the case in physical vapor deposition, or using the effect of collisional processes, as in the case of sputter deposition, for example. In some forms of deposition, a process, such as laser ablation, which creates microparticles that typically consist of one or more atoms, may replace atomization; the number of atoms per particle may be in the thousands or more. The atoms or particles of the source material are then deposited on a substrate or mandrel to directly form the desired object. In other deposition methodologies, chemical reactions between ambient gas introduced into the vacuum chamber, i.e., the gas source, and the deposited atoms and/or particles are part of the deposition process. The deposited material includes compound species that are formed due to the reaction of the solid source and the gas source, such as in the case of chemical vapor deposition. In most cases, the deposited material is then either partially or completely removed from the substrate, to form the desired product.
The rate of film growth is a significant parameter of vacuum deposition processes. In order to deposit materials that can be compared in functionality with wrought metal products, deposition rates in excess of 1 micrometers/hour are a must and indeed rates as high as 100 micrometers per hour are desirable. These are high deposition rates and it is known that at such rates the deposits always have a columnar structure. Depending on other deposition parameters, and most importantly on the substrate temperature, the columns may be amorphous or crystalline but at such high deposition rates microcrystalline structure development can be expected at best. The difficulty is that the columns provide a mechanically weak structure in which crack propagation can occur uninhibited across the whole thickness of the deposit.
A special advantage of vacuum deposition technologies is that it is possible to deposit layered materials and thus films possessing exceptional qualities may be produced (cf., H. Holleck, V. Schier: “Multilayer PVD coatings for wear protection”, Surface and Coatings Technology, Vol. 76–77 (1995) pp. 328–336). Layered materials, such as superstructures or multilayers, are commonly deposited to take advantage of some chemical, electronic, or optical property of the material as a coating; a common example is an antireflective coating on an optical lens.
It has not been recognized until relatively recently that multilayer coatings may have improved mechanical properties compared with similar coatings made of a single layer. The improved mechanical properties may be due to the ability of the interface between the layers to relieve stress. This stress relief occurs if the interface provides a slide plane, is plastic, or may delaminate locally. This property of multilayer films has been recognized in regard with their hardness but this recognition has not been translated to other mechanical properties that are significant for metal products that may be used in application where they replace wrought metal parts.
A technological step that interrupts the film growth results in discontinuous columns and prevents crack propagation across the entire film thickness. In this sense, it is not necessary that the structure consist of a multiplicity of chemically distinct layers, as it is common in the case of thin film technology where multilayers are used. Such chemical differences may be useful and may contribute to improved properties of the materials.
As used in this application a “layer” is intended to mean a substantially uniform material limited by interfaces between it and adjacent other substantially homogeneous layers, substrate, or environment. The interface region between adjacent layers is an inhomogeneous region in which extensive thermodynamic parameters may change. Different layers are not necessarily characterized by different values of the extensive thermodynamic parameters but at the interface, there is a local change at least in some parameters. For example, the interface between two steel layers that are identical in composition and microstructure may be characterized by a high local concentration of grain boundaries due to an interruption of the film growth process. Thus, the interface between layers is not necessarily different in chemical composition if it is different in structure.
It is necessary to provide for good adhesion between the layers and this is usually achieved by providing for a relatively broad interface region rather than for an abrupt interface. The width of the interface region may be defined as the range within which extensive thermodynamic parameters change. This range can depend on the interface area considered and it may mean the extent of interface microroughness. In other words, adhesion may be promoted by increased interface microroughness between adjacent layers.
By providing for a layered structure, the inventive materials consist of a controlled maximum size of grains and columns as extended defects in the direction of the film growth (perpendicular to the layers). This limit of the grain or defect size results in materials that have increased mechanical strength and particularly increased toughness compared to their non-laminated counterparts, both deposited and wrought materials. In addition, limiting the extent to which defects and grain boundaries reach across the laminate, corrosion resistance is also improved.
Laminated materials will have additional advantages when chemical compositions of the layers are chosen to achieve special properties. For example, a radiopaque material such as tantalum may form one layer of a structure while other layers are chosen to provide the material with necessary mechanical and other properties.
Heretofore, however, conventional implantable valves have traditionally been fabricated of rigid metal or synthetic materials, or have been fabricated of pliant synthetic polymeric materials, each of which involved both hemodynamic and physiological complications.